Polycrystalline diamond compact (PDC) drill bits are set with PDC cutters mounted to bit blades. Many methods for defining the setting patterns for such PDC cutters are known in the art. The goals to be achieved with respect to any PDC cutter setting pattern include: enhancing the force balancing of the drill bit; improving the cleaning of the bit face; evening out the wear of the cutters across the bit face; improving the durability of the bit; and achieving improved rates of penetration by more effectively attacking the rock to be drilled.
Reference is now made to FIG. 1. One commonly used pattern for setting the locations of PDC cutters is referred to as the “single set” method. In the single set method, each PDC cutter 20 that is positioned across the face of the bit is given a unique radial position measured from the center axis 22 of the bit outwards towards the gage 24. One commonly utilized technique for implementing a single set pattern is to define a spiral function 26 originating at the bit center axis 22 and then place individual PDC cutters at points 28 where the spiral function intersects each blade 30 location. The spiral-blade intersection points 28 will each be located at a distinct radial distance from the bit axis 22. FIG. 1 shows a cutter layout diagram for an exemplary implementation of the single set method to position cutters on a bit with six blades 30.
Reference is now made to FIG. 2. Another commonly used pattern for setting the locations of PDC cutters is referred to as the “plural set” method. In the plural set method (also known as “redundant cutter” or “tracking cutter” method), PDC cutters 20 are deployed in sets 32 containing two or more cutters each, wherein the cutters of a given set are positioned at a same radial distance 34 from the bit axis, but are located on different blades 30. The leading cutter in the set 32 is referred to as a “primary” cutter (P) (positioned, for example, on a primary blade), and the trailing cutter in the set 32 is referred to as the “secondary” cutter (S) (positioned, for example, on a secondary blade). The included primary cutters P are typically set using the single set method described above and illustrated in FIG. 1. Because of the reduced area near the center of the bit face not every PDC cutter on the bit is assured to be a member of a set positioned at the same radius, but the majority of the included cutters do belong to a set. FIG. 2 shows a cutter layout diagram for an exemplary implementation of the plural set method to position cutters on a bit with six blades 30. In one plural set pattern, as shown in FIG. 2, the cutters 20 are distributed across the bit face such that the cutters in each set 22 (at the same radius 34) are located on adjacent blades 30. In an alternative plural set pattern, not shown in FIG. 2, the cutters 20 are distributed across the bit face such that the cutters in each set 32 (at the same radius 34) are located example, on non-adjacent blades 30 (for example, blades located on the radially opposite side of the bit).
Reference is now made to FIG. 3. Attempts have been made to improve shoulder durability of drill bits through the use of a plural set pattern using backup cutters. In this method, PDC cutters 20 are deployed in sets 32 containing two or more cutters each, wherein the cutters of a given set are positioned at a same radial distance 34 from the bit axis. However, unlike the configuration of FIG. 2 where the cutters 20 of a set 32 are provided on different blades 30, the cutters 20 of a set 32 in FIG. 3 are provided on the same blade 30. The leading cutter in the set 32 is referred to as a “primary” cutter (P) (provided, for example, on a primary row of cutters), and the trailing cutter in the set 32 is referred to as the “backup” cutter (B) (provided, for example, on a backup row of cutters). The included primary cutters P are typically set using the single set method described above and illustrated in FIG. 1. Because of the reduced area near the center of the bit face not every PDC cutter on the bit is assured to be a member of a set positioned at the same radius, but the majority of the included cutters do belong to a set. FIG. 3 shows a cutter layout diagram for an exemplary implementation of the plural set method with backup cutters to position cutters on a bit with six blades 30. This cutter configuration is designed increase diamond volume while improving primary cutting structure life. The trailing backup cutters B do not function during drilling operations to destroy rock or change the work of the primary cutters P (unless the primary cutter wears in use or the backup cutter has an exposure placed on or near the bit profile defined by the primary cutters).
While the foregoing prior art patterns and methods for setting the locations of PDC cutters provide drill bits with satisfactory performance, it is nonetheless recognized that there is room for improvement, especially in connection with providing a drill bit having better shoulder cutting ability. The present invention proposes a pattern and method for setting the locations of PDC cutters in a drill bit that provides for improved shoulder durability.